Phase shifts with 'all pass' filters - understanding sought!

[jcx]

A linear and time-invariant system having the inverse causal and stable, as well.

Pretty much every linear, stable circuit that you can build.

a liitle over optimistic there (typing fingers sometimes run ahead of memory)

the all pass circuits under discussion are not minimum phase

the all pass circuit illustrates a common way to get non-minimum phase behavior - add parallel minimum phase transfer function paths together - if you get a right hand plane zero (a cancellation at some frequency) then the system doesn't have a stable inverse (the inverse of positive RHZ is a RH pole)

any circuit with a delay but flat frequency response like a transmission line is not minimum phase

minimum phase circuits follow Bode's Integral of Phase - amplitude/ and phase response are co-determinant


distributed mechanical systems with physically separated sensing and control are often limited by the non-minimum phase response from control actuator to sensor

 

[syn08]

a liitle over optimistic there
any circuit with a delay but flat frequency response like a transmission line is not minimum phase

True, no stable causal inverse. The all pass filter inverse does not have the poles in the unity circle, one of the poles is at oo.

http://ocw.mit.edu/NR/rdonlyres/BF0CB954-1B09-4CF7-81B2-B50FDEDB7F49/0/lec03.pdf

 

[kenpeter]

Back to PLL for a sec...

Suppose your music pickup is the bridge, and the sustainer is at the neck:
There is no reason to listen at one pickup and play back at the other. The
phase shift between is impossible. But maybe PLL offers an easier way?
Use the same sustaining coil to sync the PLL.

If we only strum a sustaining pulse every 5 zero crossings at some odd
subharmonic of the fundamental tone... This way the phase follows the
string behavior by vote of 4:1 edges, and not drift away with the PLL
circuit's influence feeding too much upon itself.

 

[HankMcSpank]

An old thread but revisting to ask if such software/calculators/formulas exist which will tell me this...

Online All pass filters abound on the net, that calculates the components required for a 90 degree phase shift for nominated given frequency - but does anything exist that will tell me what the phase shift for other frequencies is for that particular all pass.

Eg lets say a given allpass is configured with a capacitor & resistor to give a 90 degree phase shift at 220Hz - does anthing exist which will tell me what the actual phase shift is for say 330hz for that same all pass configuration?

 

[cbdb]

Do you know how to use LTSpice?

Is this an acoustic guitar, or a clean electric? Cause theres easier ways to get sustain out of an electric guitar (acoustic feedback ie.)

 

[HankMcSpank]

I've used sims...though I'm not particularly familiar with LTSpice - is there a plugin app that LTSpice has that would help me?

Taking my needs one further I would like to know what components to use for wrt an allpass to give a specific phase shift for a specific frequency (most allpass calculators I've seen only show components for a 90 degree phase shift for a nominated frequency)

It's an electric guitar - acoustic feedback may be easier to achieve (though I'd argue not particularly easy to 'master/control')...but it's not ideal for those living in apartments who want infinite-esque sustain at 3.00am!